Method of forming wood and formed wood product



Nov. 1, 1966 c. SCHUERCH 3,282,313

METHOD OF romaine woon AND FORMED woon PRODUCT Filed Nov 24, 1964 Cutting And Sizing The Article To Be Worked impregnating The Article With Liquid Ammdnio Bending The Article lmpressing The Article To The Desired Shupe' With A Surface Design Evclporciting The Ammonia From The Shaped Or Impressed Article INVENTOR CONRAD SCHUERCH ATTORNEYS United States Patent 3,282,313 METHQD 0F FORMING WOOD AND FORMED WOOD PRODUCT Conrad Schuerch, Syracuse, N.Y., assignor to Research Corporation, New York, N.Y., a non-profit corporation of New York Filed Nov. 24, 1964, Ser. No. 413,551 7 Claims. (Cl. 144-327) This application is a continuation-in-part of my application Serial No. 337,439, filed January 13, 1964, and now abandoned.

This invention relates to an improved process for forming wood or wood products and to the product formed by the process. The term forming is intended to include bending, pressing, stamping, shaping, shaving, fibrillizing and the like of wood and wood products whereby the surfaceconfiguration, contour, texture or bulk thereof is modified.

The usual method of bending or forming wood is to first subject the wood to a boiling or steaming treatment, usually under pressure, whereby the wood becomes impregnate-d with moisture. Various agents are often added to the water to improve its wood penetrating properties. The wet impregnated wood is shaped under pressure and then dried in individual molds or clamps, for an extended period of time, frequently not less than 48 hours. Evidently, this procedure is time-consuming and requires a great number of molds and considerable storage space. It has the added disadvantage that the physical and chemical properties of the treated wood undergo material changes, due to the prolonged treatment with hot water or steam.

It is an object of this invention to provide a process for the forming of wood which does not require the treatment of the wood being formed with hot water or steam.

It is a further object of the present invention to provide an improved process for the bending of wood which does not require the use of molds.

It is still another object of this invention to provide a process for the forming of wood which does not require the application of heat or more than a minimum of pressure to the wood being bent, impressed or otherwise formed.

The invention is based on the discovery that the foregoing objectives can be achieved by impregnating the wood to be formed with substantially anhydrous ammonia, and comprises a process for modifying wood which includes the steps of impregnating wood with substantially anhydrous ammonia, forming the impregnated wood to the desired configuration as by bending, pressing, stamping, shaving, breaking or fibrillizing the impregnated wood or by subjecting the impregnated wood to two or more such operations and removing the ammonia from the wood.

Aqueous solutions of ammonia, or ammonium hydroxide, and gaseous ammonia at ordinary temperatures .and at atmospheric pressure are not effective to accomplish the purposes of the invention. The amount of ammonia absorbed by the wood in the impregnating operation should be substantially greater than the amount molecularly equal to the moisture content of the wood being treated. This may be accomplished by immersing the wood in liquid ammonia or by subjecting it to the action of anhydrous gaseous ammonia at a pressure at least as high as the equilibrium vapor pressure of liquid ammonia at the temperature of treatment.

A preferred method of treating thick pieces of wood comprises evacuating the wood and passing ammonia into the evacuated chamber to replace the air in the wood with ammonia. This process may be repeated sev- 3,282,313 Patented Nov. 1, 1966 eral times. The wood is then subjected to the action of anhydrous ammonia in the liquid or gaseous form at a pressure at least equal to the vapor pressure of liquid ammonia at the temperature of treatment.

In general, the amount of ammonia absorbed by the wood in the impregnation operation should be at least about 10% by weight of the dry wood in excess of the amount molecularly equivalent to the moisture content of the wood. Preferably the amount of ammonia absorbed should be at least about nine times the moisture content of the wood.

The principles of the invention will be more fully described with reference to the accompanying flow diagram showing illustrative forms of the process of the invention.

Referring to the diagram, an article or piece of wood to be worked is first cut or otherwise prepared so as to have the desired size. The Work piece is then impregnated with substantially anhydrous ammonia by immersion in liquid ammonia. The ammonia impregnated workpiece is then bent to the desired shape or alternatively has a surface design impressed thereon. The permanently shaped or impressed article is obtained on evaporating the ammonia.

Accordingly, one aspect of the invention is a process for bending wood comprising the steps of impregnating a wooden article to be bent, bending the impregnated article to the desired shape and evaporating the ammonia from the bent article thereby fixing the shape of the article.

Another aspect of the invention comprises a process for impressing designs on wood which includes the steps of impregnating the wooden article to be impressed with substantially anhydrous ammonia, impressing the design into the said article and evaporating the ammonia from the impressed article.

A further aspect of the invention comprises a process for plasticizing wood for mastication or fibrillization by impregnating wood with substantially anhydrous ammonia. Mechanical disintegration of the impregnated wood may be carried out in liquid ammonia or in another liquid medium such as water.

Anhydrous ammonia has unique solvent properties which allow it to penetrate the lignin fraction of plant cell Walls and to penetrate the crystal lattice of cellulose. Ammonia is a compound unlikely to produce deep seated changes in the structure of wood components except for the ammonolysis of occasional ester groups. The absorbed ammonia is readily recoverable from the impregnated wood product by evaporation and, therefore, is particularly suited for the reversible plasticization of wood preparatory to bending or to the impressing of designs thereon or otherwise forming the Wood.

The key step of the process consists of impregnating with substantially anhydrous ammonia the wood or wooden article to be formed. The exact temperature of the ammonia employed is not critical but the wood remains somewhat brittle if working is attempted at too low a temperature. Preferably, the impregnation is carried out under conditions of temperature and pressure effective to maintain the ammonia in the liquid state. The forming operations may be carried out under sufficient pressure to maintain the ammonia in the liquid state at the temperature of the operation but this is not necessary. However, the forming operation should be completed before all of the ammonia escapes from the impregnated wood and the wood resumes its rigidity.

Sufficient time is allowed for the article being worked to become thoroughly impregnated with ammonia. N0 precise time can be specified for the impregnation step as it will naturally be dependent on the nature and thickness of the wood. being formed. The thoroughly impregnated article can be bent to the desired shape by suitable pressure or have a surface configuration impressed thereon by means of a die. After forming, the ammonia is evaporated from the body of the wood, either by warming or by release of pressure or both. The ammonia, with very little loss, can be readily recovered for 18-11156 in the process.

It is postulated that the mechanism of the wood plasticization by anhydrous ammonia is as follows: anhydrous ammonia is a strong hydrogen bonding, low molecular weight solvent which penetrates not only into the amorphous areas of the fiber cell wall but also into the crystal lattice of cellulose and into the phenolic lignin binding material. Some of the hydrogen bonds responsible for the rigidity of the wood are displaced by the ammonia. The resultant softening of the fiber structure permits molecules to slip by each other when subjected to moderate tension or compression or both. As the ammonia evaporates, hydrogen bonds are again formed between the polymer chains but not necessarily at the same locations or between the same microfibrils. The resultant hydrogen bonding cross links the Wooden structure which again becomes rigid.

There is no tendency in the formed article to return to its original shape. However, the ammonia treatment process is accompanied by some lateral and longitudinal shrinkage and by changes in moisture regain characteristics of the wood. Qualitatively, the mechanical properties of the treated wood approximate those of the original untreated wood and changes in such properties are minimized by the shortest effective treatment time.

Wood plasticization by anhydrous ammonia has possible application not only in the bending, impressing, working and forming of wood for the production of deeorative items, corrugated structures for core stock, furniture and the like, but is also effective as a plasticization method prior to mechanical disintegration. Since the vapor pressure of liquid ammonia is only 150 pounds per square inch at room temperature, mechanical disintegration or fibrillization under such a moderate pressure involves no substantial ditficulty.

The process of the invention is further illustrated by the following examples:

Strips of birch veneer inch in thickness, inch in width and 6 inches in length were covered with liquid ammonia. In one hour or less, usually about 20 minutes, the veneer was completely impregnated and softened and could be bent by fingertip pressure into circles, double S-shaped curves, helices (1% turns) or spirals 1 /2 turns). The shaped articles retained their form without clamping or other control merely by permitting them to lie on a bench top until the ammonia diffused out. Alternatively, the ammonia could be driven off with heat. A spiral with an internal diameter of curvature of inch showed no checking or failure. Soaking of the bent strips in water did not result in their return to their original shape.

Samples of pine, oak and basswood behaved in a similar manner on treatment with liquid ammonia. Specifically, a piece of oak A inch by /2 inch by 1%. inch was bent by fingertip pressure. Plasticized pine was readily stripped off a block with a knife parallel to the grain and remained as complex curved shavings inch or more thick.

Treating wood with liquid ammonia is critically different from treating wood with gaseous ammonia at atmospheric pressure or with aqueous ammonia. When aqueous ammonia is employed in place of liquid ammonia, the maximum amount of bending obtained on the birch veneer strips described above is roughly that sufficient to bring the wood to the shape of a bracelet. This shape can be produced only if the wood is clamped firmly into position and allowed to dry while still under tension; the wet WOOd 51165 to spring back into its original position. In no case using aqueous ammonia could the complex forms indicated above, i.e., helix, spiral, double S-shaped, be prepared.

Various strips of wood were exposed to a stream of gaseous ammonia for one, two, three, four, or five hours. It was noted that the amount of plasticity induced in the wood was substantially greater with the shorter treatment periods than with the longer treatment periods. An explanation for this seemingly reverse effect is that the wood near the beginning of the treatment contains approximately 7% moisture which absorbs ammonia. The small amount of aqueous ammonia so produced partially plasticizes the wood, with results, at the very best, comparable to those obtained in the previous paragraph. When the strips were treated with a stream of ammonia for longer periods of time, a stripping of the moisture from the wood resulted in a progressive lessening of its limited plasticity.

The amount of ammonia absorbed by the wood treated by the process of the invention is substantially greater than when treated with gaseous ammonia at atmospheric pressure. When immersed in liquid ammonia, the wood may contain as much as 60-70% by weight of ammonia and at the time of bending under atmospheric pressure may contain in excess of 10% by weight of absorbed ammonia. Gaseous ammonia at atmospheric pressure does not penetrate into the wood substance to any great extent, especially if the wood be dry.

Further evidence of the difference in mechanism between the wood bent under the influence of liquid ammonia and that bent after treatment with aqueous ammonia is found by testing their permanent set on treatment with water. A strip of wood veneer was bent into a circle after plasticization with aqueous ammonia and dried in that position with clamping. After being treated with hot water, the circular veneer strip became leathery and on drying regained almost its original form. This demonstrates that the aqueous ammonia process resulted in little or no change in the permanent set or fixing of the bent shape. On the other hand, when veneer strips were impregnated with liquid ammonia and bent to a gentle curve or to a spiral of approximately one and one-half turns a new permanent set resulted. This was shown by treating these stri s with hot water and drying them in an oven in exactly the same fashion as the aqueous ammonia treated strips. These strips exhibited a considerable degree of spring-back when an attempt was made to make them lie flat preparatory to drying. The dried strips did not revert to their original flat conditions but to the new bent form which was the new normal position for the wood strips. A change in the permanent set was induced only by means of the liquid ammonia treat-ment. A comparable treatment of the wood strips with aqueous ammonia or gaseous ammonia at atmospheric pressures resulted in far less plasticity, more tendency for the wood to spring back to the original form and less permanent set.

When dry birch veneer strips similar to those used in the foregoing example of treatment with liquid ammonia were impregnated with gaseous ammonia in an autoclave at ambient temperature and at the vapor pressure of liquid ammonia at that temperature, for example, about p.s.i.g. at 68 F., for three hours or longer, plasticity developed, comparable but somewhat inferior to that obtained by immersion in liquid ammonia for periods of an hour or less.

The absorption of ammonia by the wood can be facilitated by evacuating the woo-d before subjecting it to impregnation with ammonia, or by several successive evacuations and admission of gaseous ammonia to the evacuated wood to sweep out air before the impregnation. Instead of sweeping out the air with ammonia, it may be swept out with carbon dioxide, which in the impregnation step will combine with the ammonia creating a lowered internal pressure facilitating the penetration of the ammonia.

The ammonia may also be leached from the impregnated wood after the forming operation by immersing it in water rather than by permitting the ammonia to evaporate.

The overall mechanism of the ammonia treatment of the invention involves the breaking of hydrogen bonds within both the amorphous and the crystalline regions of the polysaccharide cell wall and within the lignin matrix. The ammonia softens the fiber structure and permits macromolecules to flow past one another when subjected to moderate tension or compression. When the ammonia evaporates, new hydrogen bonds are formed at new positions and very probably also to some degree between different microfi brils. As a result of the new hydrogen bond crosslinking, a substantial degree of permanent set is obtained which is not affected by treating with hot water and redrying. Such effects are not obtained with aqueous or gaseous ammonia at atmospheric pressure.

The plasticizing effect of ammonia on wood makes the wood receptive to the impression of a desired surface oonfigur'ation thereon. For example, compression of wood plasticized with liquid ammonia, as by cold compression in a Carver press, results in conversion of the wood to a much harder, mar-resistant, glossy-surfaced product. If the compression is carried out with the application of heat further collapse of the cell walls and disappearance of the lumens can render veneer layers translucent.

The foregoing discussion and examples have been presented for the purpose of illustrating the invention. The

invention is not dependent on any particular theory of the mode of operation or limited to the specific methods illustrated.

I claim:

1. A process for modifying wood which includes the steps of impregnating the wood with substantially anhydrous ammonia in an amount equal to at least 10% by weight of the dry wood in excess of the amount molecularly equivalent to the moisture content of the wood, forming the impregnated wood to a desired configuration and removing t=he ammonia from the formed wood.

2. A process as defined in claim 1 wherein the impregnation of the wood is effected by immersing it in liquid ammonia.

3. A process as defined in claim 1 wherein the impregnation of the wood is effected by subjecting the wood to ammonia gas at a superatmospheric pressure.

4. A process as defined in claim 1 wherein the forming step comprises a bending operation.

5. A process as defined in claim 1 wherein the forming step comprises an impressing operation.

6. A process as defined in claim -1 wherein the forming step comprises .a compressing operation.

7. A process as defined in claim 1 wherein the forming step comprises mechanical disintegration of the impregnated Wood.

References Cited by the Examiner UNITED STATES PATENTS 1,628,918 5/1927 Schmidt 144-327 WILLIAM W. DYER, IR., Primary Examiner.

R. J. ZLOTNIK, Assistant Examiner. 

1. A PROCESS FOR MODIFYING WOOD WHICH INCLUDES THE STEPS OF IMPREGNATING THE WOOD WITH SUBSTANTIALLY ANHYDROUS AMMONIA IN AN AMOUNT EQUAL TO AT LEAST 10% BY WEIGHT OF THE DRY WOOD IN EXCESS OF THE AMOUNT MOLECULARLY EQUIVALENT TO THE MOISTURE CONTENT OF THE WOOD, FORMING 